The present invention relates to the improvement of the structure of a filter, which has a plurality of resonators with a center conductor covered by a dielectric body.
It is well known that a conductive line with the length .lambda./4 (where .lambda. is a wavelength) operates as a resonator when one end of the line is grounded and the other end of the line is left in the air, or free standing. When a plurality of that kind of resonators are positioned and the adjacent resonators are electrically and/or magnetically coupled with each other, a small sized filter can be obtained. That kind of filter is called a distributed constant type filter, or a comb-line type filter. That kind of filter is utilized in particular in the microwave band for the use of a mobile communication.
Also, a filter with a plurality of resonators which has a center conductor covered with a dielectric body has been known. That kind of filter can be smaller in size than that which has no dielectric body covering a center conductor. The shortening ratio .alpha. of the wavelength by the dielectric body is EQU .alpha.=1/.sqroot..epsilon..sub.e
where .epsilon..sub.e is the effective dielectric constant which depends upon the structure of a line, the distribution of the electromagnetic field, and that effective dielectric constant is usually smaller than the relative dielectric constant .epsilon..sub.s of the dielectric body itself.
The disadvantage of those prior distributed constant type filter is that the characteristics of a filter cannot be adjusted after the filter is manufactured since those characteristics are defined by the structure and/or the dimension of the filter. Therefore, when there is a small manufacturing error in size of a filter, the characteristics of the filter are not satisfactory.